It is a recent general tendency to produce light and small optical elements for portable devices such as digital cameras and mobile telephones by utilizing aspherical lenses produced by press molding. If it is attempted to produce such aspherical lenses by conventional grinding and polishing, it requires many expensive and complex processings A method therefore has recently been used according to which a preform which is obtained by dripping from molten glass or grinding and polishing sheet glass is formed directly to lenses by using a highly precision-processed mold. Since it is not necessary to grind and polish lenses obtained by this method, the lenses can be manufactured at a low cost and within a short period of time. This forming method is called “glass molding” and studies and developments of this method have been made actively in recent years with resulting increase in aspherical lenses of optical elements made by glass molding. In addition to such application to aspherical lenses, glass molding is also attracting attention as a technique capable of forming articles of a fine shape accurately.
As glass used for glass molding, a glass which can be softened at a lower temperature is sought by reason of heat resistance of a mold used for glass molding. The glass transformation temperature Tg of prior art glasses comprising SiO2, B2O3, L2O3 and Li2O, however, generally exceeds 600° C. and no glass of this type of composition that sufficiently satisfies the heat resisting property of the mold used for glass molding has been provided in the past.
There is also a method of dripping molten glass from a glass flowing tube, receiving the dripping glass by a mold and cooling it to produce a glass gob. In the following description, a glass gob obtained by receiving molten glass dripped from a nozzle such as a platinum nozzle by a mold and cooling the received glass is simply called “gob”. The obtained gob may be used directly as a preform or as a lens or may be ground and polished to a spherical or lens shape and used as a lens or a preform. In case a gob is used directly as a lens or a preform, care must be taken for surface roughness as well as for preventing flaws and dirt of the lens or preform.
A typical example of the method for producing such gob is disclosed by Japanese Patent Application Laid-open Publication No. Hei 6-122526 and Japanese Patent No. 2798208. In this method, molten glass tends to become elliptical in vertical section due to its self weight in a period of time from dripping of the molten glass to solidifying of the glass by cooling. Particularly in a case where a gob of a spherical shape having a diameter of a sphere exceeding 3 mm is to be obtained, the curvature of the upper surface of the gob tends to become large. On the other hand, there is a demand not only for a flat and thin gob but also for a thick gob having a nearly spherical shape. Particularly in a case where an optical element of a relatively small size having a diameter of sphere of less than 10 mm is to be obtained, demand for a thick gob as a preform of a spherical shape is large.
A flat gob can be easily controlled by providing a device for maintaining temperature after dripping of molten glass. On the other hand, for obtaining a spherical or thick gob, a cooling device is required. However, a glass for press molding having mean linear thermal expansion coefficient exceeding 85×10−7 within a temperature range from 100° C. to 300° C. tends to cause cracking or breaking in the course of the quick cooling process due to the high expansion property of the glass. Moreover, stress tends to remain in the glass which will become a serious defect as an optical element.
It is also conceivable to obtain a gob under a high surface tension. For increasing the surface tension, it is necessary to obtain a gob at a low temperature. By reason of limitation by devitrification temperature and flow amount of glass, there is also limitation in adjustment of temperature.
Japanese Patent Application Laid-open Publication No. Hei 7-51446 discloses that a spherical gob can be produced by forming the recessed portion of a mold in the shape of a trumpet and optimizing a high pressure gas. Since, however, the gob is cooled quickly by the high pressure gas, the above described disadvantage arises.
Accordingly, it is generally practiced to cut sheet glass to cubic pieces and grind and polish the cubic pieces to obtain spherical gobs of a closely spherical shape. In this general method, however, yield of a gob from the sheet glass is 50% or lower and, therefore, it is disadvantageous from the standpoint of environmental protection and moreover it is difficult to realize production at a low cost in a short period of time. Further, in the polishing process, polishing flaws tend to take place in the gob. When such gob is used as a preform, the flaws remain in the preform after press molding and, therefore, such polishing flaws must be prevented.
As described above, various methods have been considered to obtain a gob having a nearly spherical shape but no study has been made so far to develop a gob from the standpoint of glass composition.
As glasses for press molding comprising SiO2, B2O3, La2O3 and Li2O which is an object of the present invention, there have been disclosed several glasses. For example, Japanese Patent Application Laid-open Publications No. Sho 48-61517 and No. Sho 54-3115 disclose optical glasses which comprise B2O3 and La2O3 as essential components. In these glasses, however, emphasis is placed on improvement in chemical durability and resistance to devitrification and no consideration is given to hot-forming process. Since these glasses generally have a high glass transformation temperature Tg, the forming temperature during heat forming rises above 650° C. or over with resulting deterioration in a high precision mold and, therefore, these glasses are not suitable for press molding.
Japanese Patent Application Laid-open Publication No. Sho 59-146952 discloses a B2O3—La2O3—BaO—R2O (where R is an alkali metal element) glass comprising F component. It is difficult to obtain a homogeneous glass by this glass due to adverse influence caused by evaporation of the F component.
Japanese Patent Application Laid-open Publication No. Hei 8-259257 discloses a SiO2—B2O3—ZnO—La2O3—Li2O optical glass. For obtaining a glass having an Abbe number of 50 or over by this glass, devitrification temperature must exceed 1000° C. and, therefore, it is not suitable for a large scale production.
Japanese Patent No. 3377454 discloses a SiO2—B2O3—Y2O3—La2O3—Li2O—Ta2O5 optical glass for press molding. This glass, however, generally has an exceedingly high glass transformation temperature Tg and, besides, no consideration has been given to specific gravity. This glass therefore is not suitable for an optical glass for press molding which is an object of the present invention.
Japanese Patent Application Laid-open Publication No. 2004-2178 discloses a SiO2—B2O3—La2O3—Gd2O3—Li2O optical glass for press molding. This glass, however, either has an exceedingly high glass transformation temperature Tg or has an exceedingly small specific gravity or has an exceedingly high devitrification temperature and, therefore, this glass is not suitable for an optical glass for press molding which is an object of the present invention.
Japanese Patent No. 3458462 discloses a B2O3—Li2O—La2O3—Gd2O3 optical glass for press molding. This glass, however, generally has an exceedingly high glass transformation temperature Tg or has an exceedingly small Abbe number and, therefore, this glass is not suitable for an optical glass for press molding which is an object of the present invention.
Japanese Patent No. 3059719 discloses a SiO2—B2O3—Y2O3—La2O3—Li2O optical glass. This glass, however, generally has an exceedingly high devitrification temperature or has an exceedingly small Abbe number or has an exceedingly large specific gravity and, therefore, this glass is not suitable for an optical glass for press molding which is an object of the present invention.
These publications are all insufficient in consideration as to how a low glass transformation temperature Tg should be realized. There have however been disclosed several optical glasses which have a low glass transformation temperature Tg.
For example, Japanese Patent Application Laid-open Publication No. 2003-176151 discloses an optical glass for press molding which has a low glass transformation temperature Tg. This glass, however, has an exceedingly high devitrification tendency or an exceedingly low specific gravity and, therefore, this glass is not suitable for an optical glass for press molding which is an object of the present invention.
Japanese Patent No. 2616958 discloses a SiO2— B2O3—La2O3—Gd2O3—Li2O optical glass for precision press lenses. This glass, however, generally has an exceedingly high devitrification temperature or an exceedingly small Abbe number and, therefore, is not suitable for an optical glass which is an object of the present invention.
Japanese Patent No. 3423673 discloses a glass which has an exceedingly large specific gravity or an exceedingly small Abbe number and, therefore, this glass is not suitable for an optical glass for press molding which is an object of the present invention.
Japanese Patent No. 3015078 discloses a glass for precision press having a low glass transformation temperature Tg. This glass, however, has an exceedingly high devitrification temperature or an exceedingly large Abbe number and, therefore, this glass is not suitable for an optical glass for press molding which is an object of the present invention.
It is, therefore, an object of the present invention to provide a glass which has a sufficiently low devitrification temperature and a sufficiently small specific gravity for easily obtaining a spherical product by dripping from a flow tube and which has a sufficiently low glass transformation temperature Tg which is suitable for press molding.